Force charset to UTF-8 (#14)

UTF-8 Encoding is required for event streams per spec (https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events). However, if you're running node behind a Tomcat server, the default content is set to iso-8859-1. This will throw an error in Chrome. Since UTF-8 is required, I think setting it should be part of the library.

lib/sse-channel.js

@@ -259,7 +259,7 @@ function initializeConnection(opts) {
     opts.request.socket.setNoDelay(true);
     opts.request.socket.setKeepAlive(true);
     opts.response.writeHead(200, {
-        'Content-Type': 'text/event-stream',
+        'Content-Type': 'text/event-stream;charset=UTF-8',
         'Cache-Control': 'no-cache',
         'Connection': 'keep-alive'
     });